“Tune” windows by orientation. In most situations, use low-SHGC glass for windows on east and west walls. On the north walls, solar gain isn’t a problem, so maximize thermal resistance while retaining reasonable visible light transmission. High-SHGC windows on south walls should be shaded during summer months with overhangs or other exterior shading devices.

Match windows to the functions of the space, importance of views, and location of building. If daylighting is important, ensure high visible light transmission. If views and a lot of glass are important, use high-performance glazing to minimize the energy penalties.

Don’t rely solely on windows to control solar gain. Consider exterior shading to manage direct sunlight, so the glass doesn’t have to filter out more light than necessary. For fixed shading, overhangs on the south are the best, while on the east and west, operable exterior shade screens or plantings will do a better job blocking low-angle sun.

Invest in windows with the best U-factor you can afford, allowing for appropriate solar gain, solar control, and visible light transmission. High-performance windows cost more, but they make the space near them more comfortable and may pay for themselves with the savings that result from reduced heating and cooling loads (and the ability to install smaller and less expensive heating and cooling systems).

Ensure proper installation. Make sure proper flashing details are used. Follow manufacturers’ instructions. You can test for water leakage with a hose following installation.

Make sure hinges and closures are rugged enough to handle the extra weight of operable windows.

Rely on objective performance data, such as that found on NFRC labels and through Passive House certification. Two U.S. manufacturers have attained (or are about to attain) Passive House certification for their highest-performance windows: Alpen High Performance Products and Marvin. Using certified windows is not required for Passive House projects, but the performance standard is so stringent that high-performance windows make a huge difference.

Don’t include more windows than necessary. Windows lose more heat than insulated wall sections, and too much solar gain causes overheating. This point is clearly illustrated in the “Effects of Whole-Window U-Factor on Wall R-Value” graph, which shows how windows significantly reduce a wall’s average R-value. For a wall with R-30 insulation, for example, standard double-glazed, low-e argon windows (U-0.35) taking up 12% of the wall reduces the effective R-value for the whole wall to about R-14.

Use larger panes to minimize edge losses. Applied or internal grilles, if desired, can mimic the appearance of many small panes without compromising thermal performance as significantly.

Gang windows together to reduce framing—and corresponding thermal bridging—from multiple rough openings. Make sure the head flashing is continuous and lapped properly with a weather barrier.

For windows tuned by orientation, make sure they are labeled clearly or otherwise differentiated to ensure that they get installed in the correct spot. Some designers plan south-facing windows to be a slightly different size to ensure the correct windows are used for the orientation they are specified for.

Evaluate which windows need to be operable. Better energy performance comes from fixed-glass windows, due mainly to air leakage. Rather than double-hung windows, consider single-hung windows with fixed upper sashes to cut the leakage area by half.

Avoid all-wood windows unless the wood is well-protected from the elements. Cladding makes sense for wood windows. To keep rain off the window, consider deep roof overhangs. As an alternative to wood frames, consider fiberglass, which will expand and contract less than vinyl.

Replace existing windows if they fail or are in poor shape, or if you are working to achieve a level of home energy performance that only new windows can provide. Conditions for replacement include rotted wooden frames or sills; lead paint on surfaces that are subject to friction when windows are operated; inoperable sashes or out-of-square metal or vinyl sashes; or condensation or fogging between panes of an insulating glass unit (which indicates failed seals).

When considering replacing windows that are in good condition, evaluate whether performance-enhancing window attachments, such as storm windows, interior window panels, or interior insulating blinds, may offer a more cost-effective solution.